It took me and my son (15 yr) 4.5 hrs (add 2 more hours for pickup and delivery) to install 7 yards of mulch (3-4") at two different properties. We weeded the flower beds at the same time as we went along installing the mulch.

Mike,
My mulching fee is a fixed fee and my material, labor, pickup and delivery is included in that. For the weeding part of it I would charge my hourly rate for yardwork.
Therefore the mulching supply and install hours is seperate from the weeding hours and irrelevant in the total hours of the job.
For ex: My invoice would look something like this: